Dental curable compositions composed of a polymerizable monomer, a filler, and a polymerization initiator are called composite resins, and are dental materials most widely used today as materials for restoring lost portions of teeth or carious teeth. The composite resins, when in the form of a cured product having undergone polymerization and curing, are required to have characteristics such as mechanical strength sufficient for substitution for natural teeth, polishability for obtaining gloss comparable to that of natural teeth, and gloss retention. When in the form of a paste that has yet to be polymerized and cured, they are required to have characteristics such as formability and handling properties appropriate for the operation of filling cavities by use of a dental instrument.
In recent years, composite resins called flowable composite resins, which have high flowability before being polymerized and cured, have also been developed. The flowable composite resins are used in the form of a paste for a treatment in which a cavity is filled with the paste by injecting the paste directly through a needle having an orifice with a diameter smaller than that of the cavity and attached to the tip of a container (syringe) holding the paste. The flowable composite resins make it possible to perform the filling operation simply by feeding a paste into a cavity from a syringe, and thereby allow reduction in the treatment time. Therefore, the use of flowable composite resins is becoming increasingly widespread in clinical practice.
In order for a dental curable composition to function as a flowable composite resin, the dental curable composition is required not only to have sufficient levels of mechanical strength, polishability, and gloss retention which are required of common composite resins when it is in the form of a cured product, but also to exhibit a level of flowability characteristic of flowable composite resins when it is in the form of a paste; that is, a paste of the dental curable composition is required to have a level of consistency appropriate for discharge through a needle attached to the tip of a syringe and to exhibit a level of formability and handling properties after discharge through the needle. What has the largest influence on these required characteristics is the filler contained in the dental curable composition. However, these required characteristics are mutually related to each other. That is, changing the filler to improve one of the characteristics leads to deterioration in another of the characteristics. Therefore, it is difficult to achieve high levels of the required characteristics all together. To this end, various attempts have been made thus far.
For example, Patent Literature 1 discloses a dental curable composition mainly intended to be used as a flowable composite resin. The dental curable composition is a blend of a polymerizable monomer with two types of fillers, one of which consists of irregularly-shaped inorganic particles having an average particle diameter of 1.0 to 5.0 μm and surface-treated with a silane coupling agent having a particular structure, and the other of which consists of inorganic fine particles having an average particle diameter of 0.01 to 0.10 μm and surface-treated with a silane coupling agent having a particular structure. The dental curable composition has good formability and consistency when in the form of a paste, and has excellent mechanical strength when in the form of a cured product. As a result of study by the present inventors, however, the dental curable composition of Patent Literature 1 has been found to have room for improvement in terms of the polishability and gloss retention.
Dental materials or dental compositions usable as a composite resin and containing a combination of two types of surface-treated fillers having different particle diameters are disclosed in Patent Literature 2 to 4. However, Patent Literature 2 to 4 do not include any disclosure concerning flowable composite resins, and the materials or compositions discussed in the examples of Patent Literature 2 to 4 are those which, when in the form of a paste, do not have an appropriate level of flowability as a flowable composite resin or those which have room for improvement in terms of the polishability, gloss retention, etc.